FRANDSEN. — REACTIONS OP LIMAX MAXIMUS. 193 



be explained only by some marked difference between the individual 

 animals. The first thought is that differences in response are due to a 

 difference in size, and the facts seem to give some support to that ex- 

 planation. Most of the positively geotactic individuals were found 

 among the small and medium-sized animals, and nearly all the negative 

 animals were of large size. ^Moreover, the few indifferent individuals 

 were of medium size. This, however, was not an invariable rule. 

 Small animals were sometimes negatively geotactic and, occasionally, a 

 larjre slug would migrate earthward. 



A second, clearly important, factor is the condition of the animal's 

 mucus. As shown by the preceding experiments, animals, positively 

 geotactic when normal, became negatively geotactic when lacking in 

 an abundance of sticky slime ; e. g. animal 2, Table III., and animals 2 

 and 7, Table IV. On the other hand, in one instance (22 b), a nega- 

 tive animal, when extremely sticky, went downwards. Abundant, sticky 

 mucus is evidently connected with a downward migration, and dryness 

 seems to force the animal to take an upward direction. But these facts 

 are not enough to explain all responses. For sometimes two animals 

 of nearly the same size and in equally good condition gave different 

 geotactic responses. We must look for other differences. It will, 

 however, be necessary first to refer briefly to the form and external 

 appearance of the slug. 



~-ta.v. 



Figure 1. 



Outline of Limax maxiinus. mtl. a., anterior edge of mantle ; mtl. p., posterior 

 edge of mantle; a. to mtl. p., anterior region of body ; 7iitl. p. to p., posterior region ; 

 oc, eye ; ta. d., dorsal tentacle ; ta. v., ventral tentacle ; of. puL, pulmonary orifice. 



The slug, if we except the respiratory opening on the right side of 

 the body, is externally bilaterally symmetrical. It has no external 

 shell. There are two pairs of tentacles, — a dorsal pair bearing the 

 eyes and a smaller ventral pair. The mantle extends from the neck, 

 ventrally, to near the edge of the foot. Posteriorly, it forms a prominent 

 fold, as indicated in the figure, which may be used to separate the body 

 into an anterior and a posterior region. Observations of the animal 



VOL. XXXVII. — 13 



